The occurrence of antibiotic resistant bacteria has now become widespread. Such resistance may be transferred to sensitive bacteria by F type and other conjugative plasmids; in consequence, antibiotic resistance can rapidly spread from one cell through an entire population of pathogens. A variety of F factor mutants defective in transfer or replication have been isolated. These are being used to identify and determine the nature of F gene products. Membrane proteins of mutant male strains are being examined for products associated with the conjugative process. One protein associated with the surface exclusion region of the tra operon has been identified in membranes, and is being used as an assay for tra operon expression. In addition, lambda transducing phage carrying the F traJ gene are being used for identification of the traJ product, and examination of the conditions of traJ expression. Temperature sensitive segregation mutants are being characterized genetically and biochemically.